Joint arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. Joint arthroplasty is commonly performed for hips, knees, elbows, and other joints. The health and condition of the joint to be replaced dictate the type of prosthesis necessary to replace the natural joint.
In a total hip arthroplasty an acetabular cup is implanted in the acetabular cavity in the pelvis to replace the natural acetabulum. Replacement of the acetabulum is necessary for various joint conditions, such as when there is an inadequate articulation surface for a head or ball of a prosthetic femoral component. Total hip arthroplasty is also warranted in certain cases of Developmental Displasia Hip (DDH) where the natural acetabular cavity does not properly form to allow sufficient joint articulation.
To implant an acetabular cup, a cavity is reamed in the acetabulum. The acetabular cup is then inserted into the formed cavity and secured by mechanical means, interference fit, or by a combination thereof. The acetabular cup is positioned in the pelvis at a fixed orientation with respect to patient anatomy and should remain stable.
In cases where the acetabular cavity is not generally spherical, an oblong acetabular cup may need to be implanted. Such cases include joint conditions where an implanted acetabular cup (typically a hemispherical cup) has migrated in a superior direction and certain DDH cases. The oblong geometry of the cup compensates for the elongated acetabular cavity. One type of elongated acetabular cup has an outer contour defined by two adjacent hemispheres. An exemplary dual hemisphere acetabular cup 10 is shown in FIGS. 1A and 1B. The cup 10 has an outer surface 12 defined by a first primary hemisphere 14 merged with a secondary hemisphere 16. The primary hemisphere defines a primary face 18 and the secondary hemisphere defines a secondary face 19. The primary hemisphere 14 has a concave inner surface 17 adapted for receiving the liner capable of receiving head or ball of a femoral component.
If such an oblong acetabular cup is to rely on an interference fit alone to secure the prosthesis, it is important that the cavity be formed within precise dimensional tolerances. If a proper fit is not achieved, long term fixation of an interference fit acetabular cup in the acetabulum will not occur. However, using conventional instrumentation it is difficult to achieve the required level of precision when reaming the acetabular cavity.
To implant the oblong acetabular cup, the cavity in the acetabulum is typically formed in two discrete stages. First, the true (also known as primary or natural) acetabular cavity is formed. Generally, a surgeon aligns a conventional reamer/driver instrument with the acetabulum and reams the primary or natural acetabular cavity to a hemispherical shape. FIG. 2 shows an illustrative prior art reamer/driver instrument 20 for reaming the natural acetabular cavity 22 in the acetabulum 24. Forming the natural acetabular cavity with the reamer/driver instrument 20 is a relatively straight forward process. However, to form the false or defect cavity for an oblong cup 10, the already formed natural cavity 22 should be used as a reference point to provide a cavity that matches the outer surface 12 of the oblong cup 10.
FIG. 3 shows one type of prior art instrument, known as a basket reamer/driver 30, used to form the false acetabular cavity after reaming the true acetabular cavity. As used herein, reamer/driver refers to an instrument as a whole, while reamer refers to the rotating head for removing tissue. The device 30 includes an arcuate guide 32 for placement in the reamed natural cavity and a reamer 34 for removing bony tissue. The reamer 34 is matable with a power drill mechanism for rotating the reamer.
To form the false acetabular cavity, the surgeon first visually identifies the location where the false cavity is to be formed. The false or defect cavity will be somewhere about the periphery of the natural acetabular cavity 22 (FIG. 2). After placing the guide 32 in the reamed natural cavity, the surgeon rolls the reamer 34 in a direction toward the area to be reamed to form the false acetabular cavity. However, it is difficult to retain alignment of the instrument relative to the acetabulum as the reamer is rolled to form the defect cavity. Further, it is not readily apparent when the desired amount of bone has been removed. Thus, it is difficult to form a dual hemisphere acetabular cavity with the requisite precision for long term fixation of an oblong interference fit acetabular cup.
Another drawback associated with known basket reamer/driver instruments is the lack of modularity among the various components. In general, a cavity is reamed using sequentially larger reamers. However, each reamer is adapted for coupling to a particular reamer/driver instrument sized to receive the given reamer. Thus, as a reamer/driver provides only one sized cavity, several reamer/driver instruments are needed to prepare an acetabular cavity in the acetabulum.
It would be desirable to provide a modular reaming system for forming a compound geometry cavity that precisely conforms to the outer surface of an oblong acetabular cup.